The present invention relates to a process of manufacturing crystals through a floating zone method in which raw material granular crystals are supplied continuously to obtain an ingot of monocrystals or polycrystals (semiconductors such as silicon, metals, alloys, oxides, and other compounds), and relates to an apparatus for carrying out the process.
In recent years, semiconductor wafers, particularly silicon wafer substrates, have been extensively used in solar cells, integrated circuits (ICs) and ultra large scale integrated circuits (ULSIs). As the method of manufacturing silicon monocrystals constituting substrates of this type, there is mainly used either a Czochralski method (hereinafter referred to as "CZ method") or a floating melt method (hereinafter referred to as "FZ method"). The CZ method can make large-sized crystals easily, while the FZ method can make high-purity crystals easily. The two types of manufacturing methods are selected in accordance with the purpose of use. However, the required quality of crystals has been more rigorous with the advance of integration of circuits, and improvement of the purity of crystals and remarkable reduction of cost in accordance with mass production have been required more greatly.
FIG. 5 is a view of a conventional apparatus for manufacturing crystals by crystal growth through the FZ method. FIG. 5 shows an equilibrium state at the time of crystal growth.
In FIG. 5, the reference numeral 1 designates a grown monocrystal; 2, a melt zone in a silicon bar; 3, a polycrystalline ingot as a raw material; 4, a high frequency coil; and 9, a seed crystal. In the same drawing, the raw material polycrystalline ingot 3 which is formed so as to taper to its lower end is made to approach the high frequency coil 4 so that the lower end portion of the ingot 3 is melted by induction heating (in general, 1 to 3 MHz). In this occasion, the melt is held at the lower end portion of the ingot by surface tension. When the seed crystal 9 is moved down while it is rotated after it is brought into contact with the melt at the lower end portion (that is, after seeding), monocrystal growth occurs.
The crystal 1 thus grown up from the seed crystal 9 has an enlarged size. The melt zone 2 is formed on the upper surface of the crystal 1. When the raw material crystal ingot 3 is further moved down so that the seed crystal 9 is moved down in accordance with the movement of the raw material crystal ingot 9, the melt is supplied to the melt zone 2 so that the size of the monocrystal 1 is enlarged greatly. After the size of the monocrystal 1 reaches a predetermined value, the growing of the monocrystal 1 is continued while its size is kept in the predetermined value. FIG. 5 shows an equilibrium state in this occasion. That is, the melt in the melt zone 2 is formed so that it is connected to the upper end portion of the grown crystal 1 at the lower end portion of the raw material crystal ingot 3 and held therein. In the case, alternatively, the high frequency coil 4 may be moved upward. Further, the raw material 3 and the monocrystal 1 are rotating relative to each other.
In the conventional FZ method, the volume of the polycrystalline ingot 3 required as a raw material is substantially equal to or not less than the volume of the monocrystal 1 to be produced. An ingot having a size substantially equal to or smaller than the diameter of the monocrystal 1 is used generally. Furthermore, adjustment of the shape (diameter shape, forward end shape, and holding portion shape) of the raw material ingot 3 and cleaning (etching, cleaning with pure water, and drying) of the raw material ingot 3 are required, so that a large processing loss is produced. Furthermore, the conventional manufacturing apparatus is large-sized because it requires a chamber for storing the raw material ingot 3 (generally with the length of from 1 m to 1.5 m), a mechanism for holding the raw material ingot 3 and a mechanism for moving the raw material ingot 3.